1. Field of the Invention
The present invention relates to a communication apparatus for wireless communication.
2. Description of the Related Art
In recent years, products incorporating a wireless communication function represented by a wireless local area network (to be referred to as a “wireless LAN” hereinafter) complying with the IEEE802.11 series have become widespread. Examples of such products incorporating a wireless LAN are portable devices such as a camera and a personal digital assistant (PDA). Many portable devices are driven by a battery, and thus suppression of power consumption is an important issue for these devices.
The IEEE802.11 specification defines a power saving mode (power saving enabled) in addition to a normal mode (power saving disabled) in which a wireless LAN interface is always enabled. In the power saving mode, an intermittent process of transmitting/receiving data only under a specific condition achieves low power consumption by switching between enabling (Awake state) and disabling (Doze state) of the wireless LAN interface.
The IEEE802.11 specification defines that an apparatus receives data if there is reception data, but does not define how long the Awake state should be kept. If the Awake state is kept for a long time, it is possible to prevent a data loss or a decrease in throughput but the power consumption increases. To solve this problem, Japanese Patent Laid-Open No. 2006-310909 (literature 1) proposes the following technique.
Literature 1 discloses a control method for power saving in an infrastructure mode in which an apparatus communicates with another communication apparatus (station) via the base station (access point (AP)) of a wireless LAN. That is, according to the IEEE802.11 standard, a wireless LAN terminal in the power saving mode transits to the Awake state to receive a beacon as periodic control information of the wireless LAN from the AP. The time difference between the time when a beacon is actually received and the time when the beacon should be received from the AP is calculated, thereby estimating a beacon arriving delay time (to be referred to as a “beacon delay time” hereinafter) using the time difference. An Awake time determination unit determines an Awake state period using the beacon delay time.
The invention described in literature 1 does not consider a loss of data other than a beacon as control information. To prevent a loss of data other than a beacon, the apparatus may be kept in the Awake state for a predetermined time. A power saving effect may or may not be obtained depending on the predetermined time and a beacon interval.
A case in which a power saving effect is obtained and a case in which no power saving effect is obtained will be described with reference to FIGS. 1A and 1B, respectively. Referring to FIGS. 1A and 1B, a time during which the apparatus is kept in the Awake state to prevent a data loss is 80 ms.
According to the IEEE802.11 standard, a communication apparatus (STA) transits to the Awake state to receive a beacon from an AP, and receives a beacon. The beacon includes a TIM (traffic indication message) field indicating whether data to the STA has been buffered in the AP. The STA refers to the TIM field. If the STA recognizes that data to itself has not been buffered, it transits to the Doze state.
After a predetermined time (a time corresponding to the beacon interval) elapses, the STA transits to the Awake state again, and then receives a beacon from the AP. If the TIM filed indicates that data to the STA has been buffered in the AP (which is denoted by a reference numeral 601 in FIGS. 1A and 1B), the STA transmits a PS-Poll frame 602 to the AP, and then receives data 603 from the AP. The STA attempts to transit to the Doze state about 80 ms (the predetermined time) after the beacon is received.
In FIG. 1A, the beacon interval is 100 ms. After receiving the data 603, the STA can transit to the Doze state, thereby reducing the power consumption until a next beacon is received. On the other hand, the beacon interval is 80 ms in FIG. 1B. If the data 603 is received, the STA cannot transit to the Doze state, and remains in the Awake state to receive the next beacon, thereby disabling to suppress the power consumption. That is, a beacon transmission cycle may be different for each AP. The invention described in literature 1, however, does not consider that point.
Transiting to the Doze state after the predetermined time elapses may cause a decrease in throughput or a data loss if there is data to be received or transmitted.